This invention relates to polymer dispersions in solid form and a method for their preparation. More particularly, it relates to the preparation of polymer blends in solid form.
The use of elastomeric (i.e., rubbery) polymers as additives in blends comprising other polymers is known. Various rubbery polymers are useful as impact modifiers, flame retardants and additives conferring other properties on blends in which they are incorporated. While the dispersion of liquid additives in polymeric powders is well known and previously documented by Dahms et al. (U.S. Pat. No. 3,301,813), a uniform and fine dispersion of rubbery polymers in thermoplastics to form dry free flowing powders has not been reported.
U.S. Pat. Nos. 3,824,208, 5,153,238, 5,391,594 and 5,412,014 describe the incorporation of fillers such as silica in rubbery polymers to form compositions which exist as free-flowing particles. However, it is sometimes highly desirable to exclude inorganic materials such as silica from polymer blends or coat the surface of the silica particles with siloxane elastomers, in which case these compositions are no longer free flowing. The surface chemistry of the filler in some cases can result in degradation of the matrix polymer.
Conventional approaches for obtaining free flowing powders with elastomeric components include block copolymers, core-shell copolymers or graft copolymers with thermoplastics. Copolymerization or grafting of glassy/crystalline thermoplastic prevents agglomeration of the rubbery component and enables convenient addition of these impact modifiers as free flowing powders in extrusion equipment for melt processing. Such approaches however do not provide a cost effective solution for preparing free flowing polymeric dispersions.
It is difficult, however, to prepare homogeneous blends of rubbery polymers with other resins, owing to the relative intractabilities of said rubbery polymers and the slow progress of dispersion of said polymer in the blend. Examples of some alternative approaches for obtaining free flowing powders include mixing a dispersion of an organic thermoplastic polymer with an emulsion of a silicone resin as taught by Fuhr et al. in U.S. Pat. No. 5,100,958. This method is once again not cost effective since it involves a subsequent adjustment of pH for coagulation followed by isolation and drying of the coagulate. Another method proposed by Vaughn in U.S. Pat. No. 4,153,639 involves mixing the resin and the rubbery additive (in this case the silicone gum) in a liquid medium having a component which vaporizes readily. The liquid medium is contacted with flowing live steam in a conduit and the mixture is fed into a closed chamber from which the superheated, vaporized liquid components are removed and a particulate blend is extracted.
Other practical limitations in melt-melt blending of thermoplastics with rubbery polymers include the inability to disperse the rubber phase adequately in the thermoplastic melt using the conventional processing equipment due to excessive shear heating in the extruders and a morphological balance between drop break up (dispersion) and the subsequent coalescence of the dispersed particles.
Some applications like powder coating require the availability of the thermoplastic resin blend in a powdery form. One route for the formation of thermoplastic blend powders involves high temperature melt extrusion of the various melt components followed by grinding of the thermoplastic pellets to obtain a free flowing powder. The ability to directly form uniform thermoplastic blends with fine morphologies at lower processing temperatures can provide a direct, cost-effective and simpler process.